1. Field of the Invention.
The present invention relates generally to the field of artificial joints. More particularly, it concerns a modular hip stem prosthesis with increased stability and distal flexibility to reduce thigh pain, a removably mounted, incrementally rotatable proximal shoulder with increased rotary stability, and selectable sizing of the proximal shoulder to prevent dislocation of the hip joint and to compensate for bone loss.
2. The Background Art
It is known in the art to replace the hip joint with an artificial hip stem replacement. Numerous artificial implants are available which can be installed to replace the natural hip joint with an artificial ball and socket combination. A passage called the medullary canal is reamed or bored in the upper end of the femur. A stem portion of an artificial implant is inserted into the reamed portion of the medullary canal in a secure, seated position. A neck member extends outward and away from the stem and terminates in a spherical knob for insertion into the hip in rotational contact therewith about the three major orthoganal axes.
Implants of this type have been made as a one-piece, unibody member. A hip replacement procedure using a one-piece prosthesis involves selecting a particular hip stem implant from a set of implants in order to obtain the proper size and configuration. However, the anatomy of the relevant bones is so detailed and varies so much between patients that an extensive set of prostheses is required in order to meet every conceivable anatomical condition encountered during the surgical procedure. The result is a standardized, expensive set of one-piece prostheses to cover substantially all anatomical conditions.
A partial solution to the lack of customization and affordability was attempted in the development of a series of sleeves designed to releasably receive the artificial stem. The sleeve was designed to match the contours and angles of the reamed portion of the medullary canal to provide greater rotary stability to the stem relative to the femur. Examples of sleeve and stem combinations are found in U.S. Pat. Nos. 4,790,852 and 4,846,839 (issued to Noiles on Dec. 13, 1988 and on Jul. 11, 1989, respectively). The sleeves taught therein are interchangeably mounted to the proximal end of the stem portion. The Noiles sleeve allows the surgeon to select a properly sized neck and stem piece and simply interchange the selected piece with a number of different sleeves until the proper sleeve/stem combination is found.
Although the Noiles sleeve arrangements still have restrictions, such as nonadjustability of the neck portion, the introduction of modularity into the field of hip stem prostheses has proved to be a helpful discovery. The aspect of modularity has motivated the development of hip stem prostheses having a hip stem and a mating neck assembly removably mounted on the stem. Examples of such a modular hip stem prosthesis is found in U.S. Pat. No. 5,002,578 (issued to Luman on Mar. 26, 1991). The hip stem taught in the Luman patent permits the surgeon to put together a custom hip stem prosthesis from two or more modular components. Before modularity was introduced into the art, an entire series of unibody implants was necessary for each particular component size. For example, if twenty different neck angles were desirable, each particular stem configuration would have to be reproduced twenty different times with the different neck sizes and configurations. If there were also twenty different stem configurations, these two variables alone would account for 400 different combination possibilities. Additional component variations would exponentially compound the number of unibody combinations. However, the advantages of modularity allow, in the above example, twenty different necks and twenty different stems, all of which are interchangeable with each other. Instead of 400 pieces, only forty pieces are necessary to achieve the same combinations.
The advancements made to date in the field of hip stem prostheses, while helpful, are fraught with disadvantages. Modularity alone fails to solve many of the problems which have persisted in the field. For example, the prior art hip stem devices fail to maximize the flexibility of the stem portion needed to eliminate hoop tension during insertion of the stem, and thigh pain. They also fail to provide spacing between the femur and the pelvis sufficient to prevent dislocation of the hip without introducing dangerous moment arm action of the neck capable of splintering and/or fracturing the femur. These prior art apparatus further fail to provide increased rotatory stability of the neck and precise, incremental adjustability of the neck version angle relative to the stem. Neither do the prior art devices provide a neck portion having a high degree of rotary adjustability which is removable from the stem without a significant loss of strength. The prior art also fails to adequately inhibit osteolysis caused by wear debris generation introduced by metal/metal interfacing.